1. Field of the Invention
The present invention relates generally to the art of fire tube boilers and more particularly to a fire tube boiler system which includes a specially designed burner and a duct and fan system for recirculating a minor portion of the flue gas to the burner. Still more specifically, the present invention relates to a fire tube boiler having substantially reduced NO.sub.x emissions.
2. Description of the Prior Art
The concept of flue gas recirculation to reduce NO.sub.x emissions has been known for many years, but to the knowledge of the present inventors, this concept has only been successfully applied to NO.sub.x pollution control in water tube boilers. In such prior art flue gas recirculation systems, a duct is typically connected to the flue stack and to a small recirculation fan. Another duct is coupled to the fan and to the combustion air inlet of the burner of the boiler. Only in recent years has the technology been employed with packaged boilers, mainly because of changes in state and local emission limits, principally in the State of California.
Depending on the type of fuel which is being burned, two types of nitrogen oxides can be formed. Fuel bound NO.sub.x is formed as a result of nitrogen being present in the fuel itself, e.g., in fuel oils. During combustion, the nitrogen is released and quickly reacts with the oxygen in the combustion air to form NO.sub.x. The reactions to produce fuel bound NO.sub.x are not particularly temperature-dependent. Thermal NO.sub.x is formed, on the other hand, when high combustion temperatures break down the nitrogen gas in the combustion supporting air to atomic nitrogen. When this occurs, the atomic nitrogen will very quickly react with oxygen to form thermal NO.sub.x.
If natural gas is employed as the boiler fuel, only thermal NO.sub.x is formed, because clean natural gas does not contain any nitrogen containing compounds. On the other hand, both thermal and fuel bound NO are formed when burning fuel oils. Moreover, the amount of fuel bound NO production in fuel oil combustion will depend on the quality of the oil. No. 6 oil, for example, will produce a considerably greater amount of fuel bound NO.sub.x than will No. 2 oil, because the former contains a greater quantity of fuel bound nitrogen.
It is also generally known that by cooling the combustion flame temperature, NO.sub.x production can be decreased. From the foregoing description, it is apparent that the effect of flame temperature reduction will be greatest when thermal NO.sub.x production is involved and will be less effective in reducing fuel bound NO.sub.x production. It follows then that flame temperature reduction by the recirculation of stack gas is most effective when the boiler is burning natural gas.
While NO.sub.x reduction in water tube package boilers has been successfully accomplished prior to the present invention, optimization of flue gas recirculation for fire tube boilers has been difficult. The technical problems with the two kinds of boilers result from the differences in the amount of combustion air which must be injected, and differences in pressure drops at various locations in the systems. Much higher combustion air injection pressures are required with fire tube boilers because the combustion gases flowing through the boiler must pass through the constricted area of the fire tubes. For example, with fire tube boilers, pressure losses of 15 inches or more are common, while with water tube boilers, the pressure drops encountered are typically 8 inches or less. Greater recirculation rates are required for firetube boilers in comparison to watertube boilers to obtain the same percentages of NO.sub.x reduction. This results because the combustion chamber for the firetube boiler is a narrow tube compared to the large volume chamber of the watertube boiler. This results in shorter residence time for the flue gases in firetube furnaces.
Several patents are known to the present inventors which relate generally to the subject of flue gas recirculation. In Leahy's U.S. Pat. No. 964,031, issued July 12, 1910 for "Liquid Hydrocarbon Burning Apparatus," a fire tube boiler is described, in which a portion of the combustion gas is directed from the exit of the combustion tubes to the burner area. Control of the amount of recirculated gas is provided by a damper at the exhaust stack, by a pair of dampers at the inlet side of the burner (to control injection through a plurality of orifices spaced along the combustion housing, and by control of the space which exists between the burner and the combustion housing). Leahy relates specifically to the burning of fuel oils and the recirculation of partially consumed combustion products to increase the efficiency of combustion. This patent does not teach or suggest the reduction of NO.sub.x by flue gas recirculation, the advantages thereof in reducing thermal NO.sub.x formation, the combustion device of the present invention, or any of the specific apparatus or controls used therewith. Moreover, since modern package boilers typically have extremely high combustion efficiencies, one skilled in the art would not look to Leahy as a patent of interest for NO.sub.x reduction.
In Engels' U.S. Pat. No. 2,110,209 issued Mar. 8, 1938 for "Furnace," the invention relates to protection of the costly combustion chamber by providing a blanket of cooler gas, for example recirculated flue gas, along the wall of the combustion chamber by injecting a flow of such cooler gas through a plurality of openings at the inlet to the chamber. The patent does not teach or suggest the use of recirculated flue gas to reduce NO.sub.x levels or the specific burner configuration of the present invention or the control systems used therewith. An external duct and a special fan are provided for the injection and proper mixing of the recirculated flue gas, which in turn, reduces NO.sub.x production.
Another system which employs recirculated flue gas is that described in Keller's U.S. Pat. No. 2,174,663 issued Oct. 3, 1939 for "Tubular Gas Heater." This system includes heat exchanger tubes which are traversed by gases to be heated, and in which preheated air is used for combustion. To prevent unduly high heating of the heat exchanger tubes, flue gases are recirculated in two paths, one flowing along those heat exchanger tubes in which adjoin the furnace wall, and another flow path which is introduced directly above the flame (the combustion chamber being oriented vertically). Apertures are provided to control the amount of recirculated gas passing in the two flow paths, the apertures being provided in a partition surrounding the combustion chamber. As previously mentioned, the patent relates to prevention of undue heating of heat exchange tubes, rather than NO.sub.x reduction, and the combustion device of the present invention and the controls used therewith are not disclosed or suggested in this patent.
Recirculated combustion gases are also utilized in Campbell's U.S. Pat. No. 2,430,101 issued Nov. 4, 1947 for "Combustion Chamber." The device described here is used in the baking industry. Recirculated exhaust gases are passed from an external duct into a casing surrounding the combustion chamber to reduce its temperature and to pick up heat therefrom. The recirculated gas then combines with the combustion gas at the outlet of the combustion chamber and flows through a radiator system used to warm the baking space. The patent does not suggest the case of recirculated flue gas for NO.sub.x reduction or the round combustion apparatus of the present invention.
Bailey discloses another recirculation system in his U.S. Pat. No. 3,741,166 issued June 26, 1973 for "Blue Flame Retention Gun Burners and Heat Exchanger Systems." In this system, a low pressure area is created by the vigorous injection of a major portion of the combustion supporting air through a vitiation zone positioned upstream of the fuel injection region. The low pressure area causes a portion of the combustion gases to be recirculated to chemically alter the combustion air before it encounters the fuel spray. The gases are cooled to below 800 F. before entering the vitiation zone. The remainder of the combustion air is injected through a plurality of diverging jets aimed toward the combustion chamber and is used to cool the fuel nozzle. The patent indicates that the system reduces localized hot spots which augment NO.sub.x production. This patent does not disclose the recirculation of flue gas or the use of the novel combustion apparatus of the present invention.
In commonly-assigned U.S. Pat. No. 4,519,773 issued on May 28, 1985 to Mark G. Parish, et al. for "Dual Cannister Gas Housing," a combustion apparatus is disclosed which may be employed for the introduction of two different fuel gases into a combustion chamber, the fuel gases having different fuel values. For example, the disclosed dual cannister housing could be used with natural gas, land fill gas, etc. Depending on the quantities and/or costs of the respective gases at any particular time, the gases can be supplied one at a time or mixed in the device of this patent to provide economical and efficient combustion. The aforementioned patent does not disclose flue gas recirculation, and in fact, teaches away from using a dual cannister concept for flue gas recirculation since both gases used in the patent have significant fuel values. As previously mentioned, modern boiler technology has led to efficiency improvement to the point that there is little, if any, fuel value in the exhaust gases. This is especially true for those modern boiler designs which include control systems for regulating fuel and air supply during start-up and operation under varying conditions.
An improved flue gas recirculation system for fire tube boilers which would provide substantial NO.sub.x reduction and which could be employed with new boilers or be added to existing boilers, would represent a substantial advance in the art.